Transistors of SiC may especially be used as switching devices in power applications since the cable turned on and off very rapidly. Such transistors made of SiC are particularly well suited for high power applications, since such applications make it possible to benefit from the superior properties of SiC in comparison with especially Si, namely the capability of SiC to function well under extreme conditions. SiC has a high thermal stability due to a large bandgap energy, such that devices fabricated from the material are able to operate at high temperatures, namely up to 1000.degree. K. Furthermore, Sic has a high thermal conductivity so that SiC devices may be arranged at a high density. SiC also has a more than five times higher breakdown field than Si, so that it is well suited as a material in high power devices operating under conditions where high voltages may occur in the blocking state of a device.
An advantage of this type of MISFET and IGBT having an inversion channel extending substantially laterally is that the charge carrier mobility in such an inversion channel is much higher than in the devices having a vertical inversion channel along a trench. This is because the trap density at the interface between the base layer and the insulating layer will be much lower with this location of the surface channel, since the surface of the base layer may be formed by epitaxial growth, whereas the vertical trench wall of a device with a vertical channel has to be formed by etching or the like giving rise to a higher concentration of traps. Another advantage of this type of device structure is the absence of reliability problems connected with the high electric field occuring at the trench corner of a transistor having such a trench.
MISFET is to be interpreted broadly and shall comprise every type of field effect transistor having an insulated gate, and accordingly also MOSFETs.
A transistor according to the introduction in the form of a MOSFET is known through U.S. Pat. No. 5,397,717. However, this transistor does not make it possible to fully benefit from the excellent property of SiC to withstand high reverse voltages in the blocking state thereof, so that it is not be suited for use in high power applications. The reason for this is that the base layer of p-type has to be low doped to keep the threshold voltage, which has to be applied on the gate for creating the inversion channel, comparatively low and by that obtain a high carrier mobility in the channel. However this low doping of the base layer will result in a depletion thereof already at a low reverse voltage, and thus this device cannot withstand high reverse voltages, which occur in high power applications.